Expandable microspheres and process for producing the same

ABSTRACT

Foamable microspheres with a foaming agent enclosed in the shell of a polymer, wherein the average particle diameter of the microspheres is within a range of 3 to 100 mum, and the coefficient of variation of the particle diameter distribution thereof is at most 1.50%, and production process of foamable microspheres by a suspension polymerization process.

TECHNICAL FIELD

The present invention relates to foarmable microspheres with a foamingagent enclosed in the shell of a polymer, and more particularly tofoamable microspheres extremely sharp in particle diameter distribution.The present invention also relates to a process for producing foamablemicrospheres extremely sharp in particle diameter distribution by asuspension polymerization process. The present invention further relatesto a process for producing foamable microspheres, by which aggregationof polymer particles formed and adhesion of scale to the wall of apolymerization vessel upon polymerization are prevented, and foamablemicrospheres even in particle shape in the form of a sphere and capableof sharply foaming to provide uniform foams can be provided. Thefoamable microspheres according to the present invention can be utilizedin a wide variety of technical fields including fields of paint and ink.

BACKGROUND ART

In recent years, foamable microspheres have been developed into uses invarious fields such as fillers for paints and plastics for the purposeof lightening weight. including the use of foaming ink. The foamablemicrospheres are generally obtained by microcapsulating a volatile,liquid foaming agent (also referred to as a physical foaming agent orvolatile expanding agent) by a thermoplastic resin. Such foamablemicrospheres have heretofore been produced by a process in which apolymerizable mixture containing at least a foaming agent and apolymerizable monomer is subjected to suspension polymerization in anaqueous dispersion medium. With the progress of the polymerizationreaction, a shell is formed by a polymer formed, thereby obtaining thefoamable microspheres containing the foaming agent encapsulated in theshell.

In the suspension polymerization process, the polymerizable mixture isgenerally added to the aqueous dispersion medium containing a dispersionstabilizer, the aqueous dispersion medium containing such a mixture isstirred and mixed to form fine droplets of the polymerizable mixture,and the resultant dispersion is then heated to conduct suspensionpolymerization. Since the polymerizable mixture forms an oil phase inthe aqueous dispersion medium, it can be formed into fine droplets bystirring and mixing. By the suspension polymerization, foamablemicrospheres having substantially the same particle diameter as thesefine droplets are formed. In the step of forming the fine particles ofthe polymerizable mixture, the stirring and mixing have heretofore beenconducted by means of a general agitating blade or a batch-wisehigh-speed, high-shear type dispersing machine. In the suspensionpolymerization process, foamable microspheres, the particle shape ofwhich is made even in the form of a sphere, can be provided by suitablyselecting a dispersion stabilizer, a polymerization aid and the like.Accordingly, foamable microspheres having properties satisfactory tosome extent can be obtained by devising suspension polymerizationconditions even when such stirring and mixing method as described aboveis adopted.

However, when application fields of foamable microspheres are enlarged,and higher performance comes to be required in each application field,the level required of the foamable microspheres is also raised. As theperformance of the foamable microspheres, it is particularly importantthat foaming is sharp, and foams uniform in shape and size can beformed. The phrase “forming is sharp” as used herein means that foaminitiating temperatures of individual particles of the foamablemicrospheres are substantially the same, and the particles initiatefoaming all at once under foaming temperature conditions. Therefore, thefoamable microspheres are required to have an extremely narrow particlediameter distribution in addition to the even particle shape in the formof a sphere. However, the foamable microspheres obtained in accordancewith the conventional process is not sufficiently sharp in particlediameter distribution and hence contain minute particles and coarseparticles in plenty based on an average particle diameter. When theparticle diameter distribution of the foamable microspheres is broad asdescribed above, foaming conditions among individual particles aredelicately different, and so foaming cannot be sharply conducted. Inaddition, when the particle diameter distribution is broad, foams ofuniform size cannot be obtained. Such a tendency is particularly markedwhen the average particle diameter of the foamable microspheres isgreat. On the other hand, when classification is conducted to narrow theparticle diameter distribution of the foamable microspheres, the processbecomes complicated, and a yield is lowered.

The above-described problems are described by specific examples. Forexample, the use of foamable microspheres broad in particle diameterdistribution as a weight-lightening agent or functionality-impartingagent for high-performance paints arises a problem that a finishedsurface becomes rough due to the presence of coarse particles. Thecoarse particles are easy to foam at a low temperature to impair sharpfoaming. The coarse particles also involves that an expansion ratiocannot be raised because the foaming agent is easy to escape. The minuteparticles involves a problem that an expansion ratio cannot be raisedbecause the content of the foaming agent therein is low. Such problemsbecome a fatal defect in use of the foamable microspheres for anextremely thin coating film capable of making the best use of thefeature thereof.

The foamable microspheres not only are incorporated into ink, paint,plastics and the like in an unformed state, but also may be used in afoamed state according to their uses. More specifically, foams (hollowplastic balloons) of the foamable microspheres are very light-weight andhence come to be used as a filler for paints, for example, for thepurpose of lightening the weight of an object to be coated, such as anautomobile. Since the foams are very light fine particles generallyhaving a bulk density of about 0.02 to 0.03 g/cm³ and an averageparticle diameter of about 20 to 200 μm, they are easy to escape out inthe air when they are taken out of a container and incorporated into abase material for a paint or the like. In addition, the foams gather onthe top of the base material upon their stirring and mixing with thebase material, and so it is very difficult to uniformly mix them.

Therefore, Japanese Patent Application Laid-Open No. 196813/1995 hasproposed a production process of non-scattering foamed microspheres(non-scattering hollow plastic balloons) in which unformed foamablemicrospheres are mixed with a plasticizer at a temperature lower thanthe foam initiating temperature of the foamable microspheres, theresultant mixture is brought into contact with another plasticizerheated to a temperature higher than the foam initiating temperature ofthe foamable microspheres to foam the formable microspheres, and thefoamed microspheres are cooled to prevent overfoaming. According to thisprocess, there are merits that {circle around (1)} the unformed foamablemicrospheres can be dispersed in the plasticizer to make them a fluidstate permitting quantitative feeding by a pump, {circle around (2)}uniform non-scattering foamed microspheres can be obtained at the sametime as the foaming of the foamable microspheres, and {circle around(3)} the amount of the plasticizer used can be very lessened comparedwith a process of simply wetting with a plasticizer. In order to adoptsuch a process, the foamable microspheres are required to have thefollowing properties:

(1) having good solvent resistance because a plasticizer is used as awetting agent;

(2) exhibiting a viscosity as low as possible when the foamablemicrospheres are dispersed in the plasticizer in order that themicrospheres can be quantitatively fed by a pump;

(3) causing sharp foaming from the viewpoints of process and productquality; and

(4) forming no aggregate upon a foaming process.

Accordingly, there is a demand for development of foamable microsphereshaving these properties (1) to (4) in combination. However, theconventional foamable microspheres are broad in particle diameterdistribution and hence cannot fully satisfy these required properties.

On the other hand, a production process of foamable microspheres by thesuspension polymerization process tends to cause problems that polymerparticles formed aggregate and scale adheres to the wall of apolymerization vessel upon polymerization. Therefore, various processesfor producing foamable microspheres by devising a polymerization aid anda dispersion stabilizer have heretofore been proposed. However, theconventional production processes have involved various problems andbeen not fully satisfactory.

For example, Japanese Patent Publication No. 26524/1967 describesunicellular particles (i.e., foamable microspheres) of a thermoplasticresinous polymer with a volatile liquid foaming agent, which becomes agaseous state at a temperature lower than the softening point of thepolymer, enclosed therein. This publication discloses a process forproducing spherical particles with the foaming agent enclosed in a shellformed of the thermoplastic resin by adding the foaming agent such as alow-boiling aliphatic hydrocarbon to a monomer, mixing an oil-solublecatalyst with the resultant monomer mixture and then adding the monomermixture to an aqueous dispersion medium containing a dispersing agentwith stirring to conduct suspension polymerization. Japanese PatentApplication Laid-Open No. 286534/1987 describes a process for producingheat-expanding microcapsules (i.e., foamable microspheres) by using apolymer obtained from a component comprising at least 80 wt. % of anitrile monomer, at most 20 wt. % of a non-nitrile monomer and acrosslinking agent to enmicrocapsulating a volatile expanding agent. Inthese conventional production processes, a polymerizable mixturecomprising a foaming agent, a polymerizable monomer and a polymerizationinitiator is subjected to suspension polymerization in an aqueousdispersion medium containing colloidal silica as a dispersion stabilizer(suspending agent), a diethanolamine-adipic acid condensation product asan auxiliary stabilizer and potassium bichromate as a polymerizationaid, thereby producing the foamable microspheres.

However, potassium bichromate used as the polymerization aid in theseconventional processes involves a problem that it has toxicity. Inaddition, when potassium bichromate is used, the resulting foamablemicrospheres are colored yellow due to a remaining chromium ion, therebyimpairing the color tone of various products comprising suchmicrospheres in an unformed or foamed state. When such yellowmicrospheres are caused to be contained in a colored product inparticular, the color tone of the product tends to become dull.

When potassium bichromate is not used upon the suspensionpolymerization, polymer particles formed show a tendency to aggregate,or a problem that a polymer formed adheres as scale to the wall of apolymerization vessel is easy to arise. When the polymer particlesaggregate, the viscosity of the suspension polymerization reactionsystem is increased to adversely affect the progress of thepolymerization reaction and the particle shape of the resulting foamablemicrospheres. When polymer scale covers the wall of the polymerizationvessel, the heat removing ability of the polymerization vessel islowered, and a yield of the foamable microspheres is reduced. Whenaggregates of the polymer particles or peeled matter from polymer scaleadhered are mixed in the foamable microspheres, a problem that afinished surface becomes rough arises when such foamable microspheresare used as, for example, a weight-lightening agent orfunctionality-imparting agent for high-performance paints, since theseaggregates and peeled matter from polymer scale are coarse particles.These coarse particles and particles in a form out of sphere causeproblems that they are easy to foam at a low temperature compared withspherical particles, and that the foaming agent is easy to escape, andso an expansion ratio cannot be raised. Such problems become a fataldefect in use of the foamable microspheres for, in particular, anextremely thin coating film capable of making the best use of thefeature thereof. In the case of their use for air spray, the clogging ofa gun and uneven coating tend to occur.

Japanese Patent Application Laid-Open No. 292643/1992 (Patent No.2584376) discloses a process for producing foamable thermoplasticmicrospheres by using, as a suspending agent (dispersion stabilizer), apowdered stabilizer insoluble in an aqueous medium at a pH that theaqueous medium has upon polymerization, such as magnesium hydroxide.This publication states that “According to this process, the powderedstabilizer can be dissolved and removed by lowering the pH of theaqueous medium after the polymerization, so that foamable microsphereshaving a clean polymer surface can be provided.” However, the problemthat polymer particles aggregate cannot be solved even by this process.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide foamablemicrospheres which have a spherical particle shape, are extremely sharpin particle diameter distribution and are capable of sharply foaming toprovide uniform foams.

Another object of the present invention is to provide a process forproducing such foamable microspheres sharp in particle diameterdistribution.

A further object of the present invention is to provide a process forproducing foamable microspheres, by which aggregation of polymerparticles formed and adhesion of scale to the wall of a polymerizationvessel upon polymerization are prevented, and foamable microspheres evenin particle shape in the form of a sphere and capable of sharply foamingto provide uniform foams can be provided.

The present inventors have carried out an extensive investigation with aview toward overcoming the above-described problems involved in theprior art. As a result, it has been found that in a process forproducing foamable microspheres with a foaming agent enclosed in theshell of a polymer formed by subjecting a polymerizable mixturecontaining at least the foaming agent and a polymerizable monomer tosuspension polymerization in an aqueous dispersion medium, the aqueousdispersion medium and the polymerizable mixture are fed to a continuoushigh-speed, high-shear type stirring and dispersing machine,continuously stirring both in the stirring and dispersing machine so asto disperse the polymerizable mixture in the aqueous dispersion medium,and the resultant dispersion is then poured into a polymerization tankto conduct suspension polymerization in the polymerization tank, wherebyfoamable microspheres having an average particle diameter within a rangeof 3 to 100 μm and an extremely sharp particle diameter distribution ofat most 1.50% in terms of the coefficient of variation of the particlediameter distribution are provided. The foamable microspheres are noveland can sharply foam due to their low contents of coarse particles andminute particles to provide uniform foams.

As a result of the extensive investigation, the present inventors havealso found that in a process for producing foamable microspheres with afoaming agent enclosed in the shell of a polymer formed by subjecting apolymerizable mixture containing at least the foaming agent and apolymerizable monomer to suspension polymerization in an aqueousdispersion medium, the suspension polymerization of the polymerizablemixture is conducted in the presence of at least one compound selectedform the group consisting of alkali metal nitrites, stannous chloride,stannic chloride, water-soluble ascorbic acids and boric acid, wherebyfoamable microspheres can be produced stably without causing aggregationof polymer particles formed upon the polymerization while preventing thepolymer formed from adhering to the wall of a polymerization vessel andefficiently removing heat generated by the polymerization. The foamablemicrospheres obtained according to this production process can sharplyfoam to provide uniform foams due to their low contents of asphericalparticles and aggregated particles.

The present invention has been led to completion on the basis of thesefindings.

According to the present invention, there are thus provided foamablemicrospheres with a foaming agent enclosed in the shell of a polymer,wherein the average particle diameter of the microspheres is within arange of 3 to 100 μm, and the coefficient of variation of the particlediameter distribution thereof is at most 1.50%.

According to the present invention, there is also provided a process forproducing foamable microspheres with a foaming agent enclosed in theshell of a polymer formed by subjecting a polymerizable mixturecontaining at least the foaming agent and a polymerizable monomer tosuspension polymerization in an aqueous dispersion medium, the processcomprising feeding the aqueous dispersion medium and the polymerizablemixture into a continuous high-speed, high-shear type stirring anddispersing machine, continuously stirring both in the stirring anddispersing machine so as to disperse the polymerizable mixture in theaqueous dispersion medium, and then pouring the resultant dispersioninto a polymerization tank to conduct suspension polymerization in thepolymerization tank.

In the step of feeding the aqueous dispersion medium and thepolymerizable mixture into the continuous high-speed, high-shear typestirring and dispersing machine, the aqueous dispersion medium and thepolymerizable mixture may preferably be continuously fed as separatestreams at a fixed ratio into the continuous high-speed, high-shear typestirring and dispersing machine. As another method, may be mentioned amethod in which the aqueous dispersion medium and the polymerizablemixture are poured into a dispersing tank, both are stirred in thedispersing tank to primarily disperse the polymerizable mixture in theaqueous dispersion medium, and the resultant primary dispersion is thenfed into the continuous high-speed, high-shear type stirring anddispersing machine.

According to the present invention, there is further provided a processfor producing foamable microspheres with a foaming agent enclosed in theshell of a polymer formed by subjecting a polymerizable mixturecontaining at least the foaming agent and a polymerizable monomer tosuspension polymerization in an aqueous dispersion medium, the processcomprising conducting the suspension polymerization of the polymerizablemixture in the presence of at least one compound selected from the groupconsisting of alkali metal nitrites, stanous chloride, stannic chloride,water-soluble ascorbic acids and boric acid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of the production process according to thepresent invention using a continuous high-speed, high-shear typestirring and dispersing machine.

FIG. 2 illustrates another example of the production process accordingto the present invention using the continuous high-speed, high-sheartype stirring and dispersing machine.

FIG. 3 illustrates an example of the conventional production processusing a batch-wise high-speed, high-shear type dispersing machine.

BEST MODE FOR CARRYING OUT THE INVENTION

Foamable Microspheres

In the foamable microspheres according to the present invention, theaverage particle diameter thereof is within a range of 3 to 100 μm, andthe coefficient of variation of the particle diameter distributionthereof is at most 1.50%. The average particle diameter of the foamablemicrospheres according to the present invention, and the averageparticle diameter of foams thereof may be both varied within wideranges, and can be suitably designed according to uses thereof. Theaverage particle diameter of the foamable microspheres according to thepresent invention is preferably within a range of 5 to 50 μm in anunformed state. The coefficient of variation of the particle diameterdistribution of the foamable microspheres according to the presentinvention is preferably at most 1.30%, more preferably at most 1.10%when sharp foaming behavior to a particularly high degree or goodsmoothness on the surface of a coating film is required. The lower limitof the coefficient of variation of the particle diameter distribution isabout 0.01%, often about 0.03%.

The coefficient of variation as referred to In the present invention isa value calculated out based on the following equations (1) and (2):$\begin{matrix}{C_{v} = {\left( {\sqrt{{\frac{1}{100}\quad {\sum\limits_{j = 1}^{n}\quad {q_{j}\quad \left( \frac{{\log \quad x_{j}} + {\log \quad x_{j + 1}}}{2} \right)^{2}}}} - \mu^{2}}/\mu} \right) \times 100}} & (1) \\{\mu = {\frac{1}{100}\quad {\sum\limits_{j = 1}^{n}\quad {q_{j}\left( \frac{{\log \quad x_{j}} + {\log \quad x_{j + 1}}}{2} \right)}}}} & (2)\end{matrix}$

wherein μ is an average value, x_(j) is a particle diameter, and q_(j)is a frequency distribution.

The content of the foaming agent in the foamable microspheres accordingto the present invention is generally 5 to 30 wt. %, preferably 10 to 25wt. %. Examples of the foaming agent include low-boiling organicsolvents and compounds decomposed by heating to generate a gas. Amongthese, the low-boiling organic solvents are preferred. The shell of thepolymer making up each of the foamable microspheres according to thepresent invention can be formed by using at least one of variouspolymerizable monomers such as acrylic esters, (meth)acrylonitrile,vinylidene chloride, vinyl chloride and styrene. Among others, the shellis preferably formed with a vinylidene chloride copolymer or a(meth)acrylonitrile copolymer in that gas barrier properties, solventresistance, heat resistance, foamability and the like are balanced withone another at a high level. According to the present invention,foamable microspheres exhibiting various foaming behaviors can beprovided by selecting the combination of the polymerizable monomersused, controlling a compositional ratio therebetween, and selecting thekind of the foaming agent.

The foamable microspheres sharp in particle diameter distributionaccording to the present invention cause sharp foaming. When the foamingstate of foamable microspheres is observed through, for example, amicroscope equipped with a hot stage while heating them, it is foundthat the foamable microspheres obtained by the production processaccording to the present invention foam at a stretch like popcorn toform uniform foams. On the other hand, foamable microspheres obtained bystirring a polymerizable monomer in an aqueous dispersion medium bymeans of a general agitating blade or a batch-wise high-speed,high-shear type dispersing machine to prepare a dispersion and thensubjecting the dispersion to suspension polymerization according to theconventional process become broad in particle diameter distributioncompared with the foamable microspheres according to the presentinvention, and some of their particles initiate foaming at a temperaturelower than the prescribed foam initiating temperature by at least 10° C.or at least 30° C. in some cases.

The conventional foamable microspheres obtained by the suspensionpolymerization is observed greatly reducing their weight due to leak ofthe foaming agent even at a temperature lower than the foam initiatingtemperature when their weight loss upon heating is determined by athermobalance (TGA). Such weight loss takes place due to the leak of thefoaming agent from the foamable microspheres before initiation offoaming. Therefore, the prescribed expansion ratio may not be achieved,and in extreme cases, no foaming occurs. When the process described inJapanese Patent Application Laid-Open No. 196813/1995 is applied to suchfoamable microspheres broad in particle diameter distribution and unevenin foaming, the viscosity of a slurry upon mixing of a mixture of thefoamable microspheres and a plasticizer increases, or partial foamingoccurs upon preheating. Since foamable microspheres expand to a degreeof 60 to 100 times by volume upon foaming, the mixture with theplasticizer loses its flowability even when foaming partially occurs, sothat its pumping becomes infeasible.

Production Process of Foamable Microspheres (I)

The first production process according to the present invention is aprocess for producing foamable microspheres with a foaming agentenclosed in the shell of a polymer formed by subjecting a polymerizablemixture containing at least the foaming agent and a polymerizablemonomer to suspension polymerization in an aqueous dispersion medium. Inthis process, the aqueous dispersion medium and the polymerizablemixture are fed into a continuous high-speed, high-shear type stirringand dispersing machine to continuously stir both in the stirring anddispersing machine so as to disperse the polymerizable mixture in theaqueous dispersion medium, and the resultant dispersion is then pouredinto a polymerization tank to conduct suspension polymerization in thepolymerization tank, thereby producing the foamable microspheres.

For example, as illustrated in FIG. 3, an aqueous dispersion medium 1and a polymerizable mixture 2 have heretofore been poured into abatch-wise high-speed, high-shear type dispersing machine 16 and stirredto disperse the polymerizable mixture in the aqueous dispersion medium,thereby forming fine droplets of the polymerizable mixture, and theresultant dispersion has then been poured into a polymerization tank 11through a line 18 by means of a pump 17, thereby conducting suspensionpolymerization in the polymerization tank. According to such aconventional process, only foamable microspheres the coefficient ofvariation of the particle diameter distribution of which exceeds 1.50%or at least 2.00% in many cases can be provided.

On the other hand, according to the production process of the presentinvention, for example, as illustrated in FIG. 1, in the step of feedingthe aqueous dispersion medium and the polymerizable mixture into thecontinuous high-speed, high-shear type stirring and dispersing machine,the aqueous dispersion medium 1 and the polymerizable mixture 2 arecontinuously fed as separate streams at a fixed ratio into thecontinuous high-speed, high-shear type stirring and dispersing machine.More specifically, the aqueous dispersion medium 1 and the polymerizablemixture 2 are stored in a storage tank 3 and a storage tank 4,respectively. The aqueous dispersion medium 1 and the polymerizablemixture 2 are fed as separate streams into the continuous high-speed,high-shear type stirring and dispersing machine 9 through a line 6 bymeans of a pump 5 and a line 8 by means of a pump 7, respectively. Afeeding ratio of the aqueous dispersion medium 1 to the polymerizablemixture 2 is generally within a range of 1:1 to 6:1, preferably 2:1 to4:1. Both are continuously stirred in the stirring and dispersingmachine 9 to disperse the polymerizable mixture in the aqueousdispersion medium, and the resultant dispersion is then poured into apolymerization tank 11 through a line 10 to conduct suspensionpolymerization in the polymerization tank 11.

According to another embodiment of the production process of the presentinvention, as illustrated in FIG. 2, in the step of feeding the aqueousdispersion medium and the polymerizable mixture into the continuoushigh-speed, high-shear type stirring and dispersing machine, the aqueousdispersion medium 1 and the polymerizable mixture 2 are poured into adispersing tank 12, and both are stirred in the dispersing tank 12 toprimarily disperse the polymerizable mixture in the aqueous dispersionmedium. The dispersing tank 12 is usually equipped with a generalagitating blade. A feeding ratio of the aqueous dispersion medium 1 tothe polymerizable mixture 2 is generally within a range of 1:1 to 6:1,preferably 2:1 to 4:1. The resultant primary dispersion obtained bystirring within the dispersing tank is fed into the continuoushigh-speed, high-shear type stirring and dispersing machine 9 through aline 14 by means of a pump 13. The primary dispersion is furtherstirring and dispersed in the stirring and dispersing machine 9, and theresultant dispersion is then poured into the polymerization tank 11through a line 15 to conduct suspension polymerization in thepolymerization tank 11.

In the production process according to the present invention, conditionssuch as the number of revolutions of the continuous high-speed,high-shear type stirring and dispersing machine and the mixing ratio ofthe aqueous dispersion medium to the polymerizable mixture are presetaccording to the desired particle diameter of the foamable microspheres.The number of revolutions of the continuous high-speed, high-shear typestirring and dispersing machine is generally selected within a range of1,400 to 14,000 rpm, preferably 2,000 to 5,000 rpm. It is consideredthat the size of droplets of the polymerizable mixture is made even bycontinuously applying high shearing force to the aqueous dispersionmedium and the polymerizable mixture by the continuous high-speed,high-shear type stirring and dispersing machine to stir them at a highspeed, and the particle diameter distribution thereof is hence narrowed.As a result, foamable microspheres having an extremely sharp particlediameter distribution of at most 1.50%, preferably at most 1.30%,particularly preferably at most 1.10%, in terms of the coefficient ofvariation of the particle diameter distribution can be provided. Thefoamable microspheres according to the present invention can solve suchinconveniences as described above to exhibit excellent variousproperties.

In the production processes according to the present invention, noparticular limitation is imposed on the foaming agent, polymerizablemonomer, other auxiliaries, etc., and those conventionally known may beused. More specifically, the production processes according to thepresent invention can be applied to the production of foamablemicrospheres of all types.

(1) Foaming Agent

The foaming agent in the present invention are generally a substancewhich becomes a gaseous state at a temperature lower than the softeningpoint of the polymer forming the shell. As such a foaming agent, ispreferred a low-boiling organic solvent, and examples thereof includelow-molecular weight hydrocarbons such as ethane, ethylene, propane,propene, n-butane, isobutane, butene, isobutene, n-pentane, isopentane,neopentane, n-hexane, heptane and petroleum ether; chlorofluorocarbonssuch as CCl₃F, CCl₂F₂, CClF₃, CClF₂—CCl₂F₂; and tetraalkylsilanes suchas tetramethyl-silane, trimethylethylsilane, trimethylisopropylsilaneand trimethyl-n-propylsilane. These compounds may be used either singlyor in any combination thereof. Among these, isobutane, n-butane,n-pentane, isopentane, n-hexane, petroleum ether and mixtures of atleast two thereof are preferred. Any compound which is decomposed byheating to become a gaseous state may also be used if desired.

(2) Polymerizable Monomer

Examples of the polymerizable monomer include acrylates such as methylacrylate, ethyl acrylate, butyl acrylate and dicyclopentenyl acrylate;methacrylates such as methyl methacrylate, ethyl methacrylate, butylmethacrylate and isobornyl methacrylate; and besides acrylonitrile,methacrylonitrile, vinylidene chloride, vinyl chloride, styrene, vinylacetate, α-methylstyrene, chloroprene, neoprene and butadiene. Thesepolymerizable monomers may be used either singly or in any combinationthereof.

In the foamable microspheres, it is preferred that the polymer forforming the shell thereof should be thermoplastic and have gas barrierproperties. From these points of view, vinylidene chloride copolymersand (meth)acrylonitrile copolymers are preferred as polymers for formingthe shell.

Examples of the vinylidene chloride copolymers include copolymersobtained by using, as polymerizable monomers, 30 to 95 wt. % ofvinylidene chloride and 5 to 70 wt. % of a monomer copolymerizabletherewith. Examples of the monomer copolymerizable with vinylidenechloride include acrylonitrile, methacrylonitrile, methacrylates,acrylates, styrene and vinyl acetate. More specifically, a monomermixture containing (a) 30 to 95 wt. % of vinylidene chloride and (b) 5to 70 wt. % of at least one monomer selected from the group consistingof acrylonitrile, methacrylonitrile, acrylates, methacrylates, styreneand vinyl acetate is preferably used as a polymerizable monomer toproduce foamable microspheres. If the copolymerizing proportion ofvinylidene chloride is lower than 30 wt. %, the gas barrier propertiesof the resulting copolymer are deteriorated. If the copolymerizingproportion is higher than 95 wt. %, the foaming temperature range of theresulting foamable microspheres becomes too low. It is hence notpreferable to use vinylidene chloride in such a low or high proportion.

As the vinylidene chloride copolymer, is more preferred a terpolymerobtained by using, as a polymerizable monomer, a monomer mixturecontaining (a) 40 to 80 wt. % of vinylidene chloride, (b1) 19 to 50 wt.% of at least one monomer selected from the group consisting ofacrylonitrile and methacrylonitrile, and (b2) 1 to 20 wt. % of at leastone monomer selected from the group consisting of acrylates andmethacrylates in that a practicable foaming temperature is easilydesigned, and a high expansion ratio is easy to achieve.

When high solvent resistance and the ability to foam at a hightemperature are desired, it is preferred that the shell is formed by acopolymer comprising (meth)acrylonitrile as a main component. Morespecifically, a monomer mixture containing (c) 51 to 95 wt. % of atleast one monomer selected from the group consisting of acrylonitrileand methacrylonitrile and (d) 5 to 49 wt. % of at least one monomerselected from the group consisting of vinylidene chloride, acrylates,methacrylates, styrene and vinyl acetate is preferably used as apolymerizable monomer to produce foamable microspheres. More preferably,the monomer mixture is that containing (c) 51 to 95 wt. % of at leastone monomer selected from the group consisting of acrylonitrile andmethacrylonitrile, (d1) 1 to 40 wt. % of vinylidene chloride, and (d2) 1to 48 wt. % of at least one monomer selected from the group consistingof acrylates and methacrylates. If the copolymerizing proportion of(meth)acrylonitrile is lower than 51 wt. %, the solvent resistance andheat resistance of the resulting foamable microspheres are deteriorated.If the copolymerizing proportion is higher than 95 wt. %, theheat-expanding ability of the resulting foamable microspheres isdeteriorated. It is hence not preferable to use (meth)acrylonitrile insuch a low or high proportion.

As a copolymer containing no vinylidene chloride, is preferred a(meth)acrylonitrile copolymer obtained by using, as a polymerizablemonomer, a monomer mixture containing (e) 70 to 95 wt. % of at least onemonomer selected from the group consisting of acrylonitrile andmethacrylonitrile, and (f) 5 to 30 wt. % of at least one monomerselected from the group consisting of acrylates and methacrylates. Morepreferably, the monomer mixture is that containing (e1) 55 to 75 wt. %of acrylonitrile, (e2) 20 to 40 wt. % of methacrylonitrile, and (f) 1 to10 wt. % of at least one monomer selected from the group consisting ofacrylates and methacrylates. Foamable microspheres excellent in gasbarrier properties, solvent resistance, heat resistance, foamability,etc. can be obtained even from such a (meth)acrylonitrile copolymer.

(3) Crosslinkable Monomer

A crosslinkable monomer may be used in combination with suchpolymerizable monomers as described above with the view toward improvingthe foaming properties and heat resistance of the resulting foamablemicrospheres. As the crosslinkable monomer, is generally used a compoundhaving at least two carbon-carbon double bonds. More specifically,examples of the crosslinkable monomer include divinylbenzene, ethyleneglycol di(meth)acrylate, triethylene glycol di(meth)acrylate, allylmethacrylate, triallyl isocyanate, triacrylformal, trimethylolpropanetri(meth)acrylate, 1,3-butylene glycol dimethacrylate andpentaerythritol tri(meth)acrylate. A proportion of the crosslinkablemonomer used is generally 0.1 to 1 wt. %, preferably 0.2 to 0.8 wt. %based on the polymerizable monomer.

(4) Polymerization Initiator

No particular limitation is imposed on a polymerization initiator, andthose generally used in this field may be used. However, an oil-solublepolymerization initiator soluble in the polymerizable monomer ispreferred.

Examples of the polymerization initiator include dialkyl peroxides,diacyl peroxides, peroxy esters, peroxydicarbonates and azo compounds.More specifically, examples thereof include dialkyl peroxides such asmethyl ethyl peroxide, di-t-butyl peroxide and dicumyl peroxide; diacylperoxide such as isobutyl peroxide, benzoyl peroxide,2,4-dichlorobenzoyl peroxide, and 3,5,5-trimethylhexanoyl peroxide;peroxy esters such as t-butyl peroxypivalate, t-hexyl peroxypivalate,t-butyl peroxyneodecanoate, t-hexyl peroxyneodecanoate,1-cyclohexyl-1-methylethyl peroxyneodecanoate, 1,1,3,3-tetramethylbutylperoxyneodecanoate, cumyl peroxyneodecanoate and(α,α-bis-neodecanoylperoxy)diisopropylbenzene; peroxydicarbonates suchas bis(4-t-butylcyclohexyl)peroxydicarbonate, di-n-propylperoxydicarbonate, diisopropyl peroxydicarbonate,di(2-ethylethylperoxy). dicarbonate, dimethoxybutyl peroxydicarbonateand di(3-methyl-3-methoxybutylperoxy)dicarbonate; and azo compounds suchas 2,2′-azobisisobutyronitrile,2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile),2,2′-azobis(2,4-dimethylvaleronitrile) and1,1′-azobis(1-cyclohexanecarbonitrile).

The polymerization initiator is generally contained in the monomermixture. However, when premature polymerization must be prevented, apart or the whole thereof may be contained in the aqueous dispersionmedium to shift it into droplets of the polymerizable mixture during orafter the formation of the droplets. The polymerization initiator isgenerally used in a proportion of 0.0001 to 3 wt. % based on the aqueousdispersion medium.

(5) Aqueous Dispersion Medium

The suspension polymerization is conducted in an aqueous dispersionmedium containing a dispersion stabilizer (suspending agent). Examplesof the dispersion stabilizer include silica, calcium phosphate,magnesium hydroxide, aluminum hydroxide, ferric hydroxide, bariumsulfate, calcium sulfate, sodium sulfate, calcium oxalate, calciumcarbonate, barium carbonate and magnesium carbonate. Besides, anauxiliary stabilizer, for example, a condensation product ofdiethanolamine and an aliphatic dicarboxylic acid, a condensationproduct of urea and formaldehyde, polyvinyl pyrrolidone, polyethyleneoxide, polyethylene-imine, tetramethylammonium hydroxide, gelatin,methyl cellulose, polyvinyl alcohol, dioctyl sulfosuccinate, sorbitanester, one of various emulsifiers, or the like, may be used. Thedispersion stabilizer is generally used in a proportion of 0.1 to 20parts by weight per 100 parts by weight of the polymerizable monomer.

The aqueous dispersion medium containing the dispersion stabilizer isgenerally prepared by mixing the dispersion stabilizer and the auxiliarystabilizer into deionized water. The pH of the aqueous phase upon thepolymerization is suitably determined according to the kinds of thedispersion stabilizer and auxiliary stabilizer used. For example, whensilica such as colloidal silica is used as the dispersion stabilizer,the polymerization is conducted in an acidic environment. In order toacidify the aqueous dispersion medium, an acid is added as needed toadjust the pH of the system to about 3 to 4. When magnesium hydroxide orcalcium phosphate is used, the polymerization is conducted in analkaline environment.

A preferable combination includes a combination of colloidal silica witha condensation product. The condensation product is preferably acondensation product of diethanolamine and an aliphatic dicarboxylicacid, particularly a condensation product of diethanolamine and adipicacid or condensation product of diethanolamine and itaconic acid. Thecondensation product is defined by an acid number thereof. Acondensation product having an acid number not lower than 60, but lowerthan 95 is preferred, with a condensation product having an acid numbernot lower than 65, but not higher than 90 being particularly preferred.When an inorganic salt such as sodium chloride or sodium sulfate isfurther added, foamable microspheres having an evener particle shape areeasy to obtain. As the inorganic salt, is preferably used common salt.

Although the amount of colloidal silica used varies according to theparticle diameter thereof, it is used in a proportion of generally 1 to20 parts by weight, preferably 2 to 10 parts by weight per 100 parts byweight of the polymerizable monomer. The condensation product is used ina proportion of generally 0.05 to 2 parts by weight per 100 parts byweight of the polymerizable monomer. The inorganic salt is used in aproportion of about 0 to 100 parts by weight per 100 parts by weight ofthe polymerizable monomer.

Another preferable combination includes a combination of colloidalsilica with a water-soluble nitrogen-containing compound. Examples ofthe nitrogen-containing compound include polyvinyl pyrrolidone,polyethylene-imine, polyoxyethylene alkylamine, polydialkylaminoalkyl(meth)acrylates typified by polydimethylaminoethyl methacrylate andpolydimethylaminoethyl acrylate, polydialkylaminoalkyl(meth)acrylamidestypified by polydimethylaminopropylacrylamide andpolydimethylaminopropylmethacrylamide, polyacrylamide, cationicpolyacrylamide, polyamine sulfone, and polyallylamine. Among these, thecombination of colloidal silica with polyvinyl pyrrolidone is preferablyused. A further preferable combination includes a combination ofmagnesium hydroxide and/or calcium phosphate with an emulsifier.

As the dispersion stabilizer, may be used colloid of a hardlywater-soluble metal hydroxide (for example, magnesium hydroxide)obtained by the reaction of a water-soluble polyvalent metallic compound(for example, magnesium chloride) with an alkali metal hydroxide (forexample, sodium hydroxide) in an aqueous phase. As the calciumphosphate, may be used a reaction product of sodium phosphate withcalcium chloride in an aqueous phase. As the emulsifier, may also beused an anionic surfactant, for example, a salt of dialkyl sulfosuccinicacid or a phosphoric ester of polyoxyethylene alkyl (allyl) ether.

At least one compound selected from the group consisting of alkali metalnitrites, stannous chloride, stannic chloride, water-soluble ascorbicacids and boric acid may also be caused to exist as a polymerization aidin the aqueous dispersion medium. When suspension polymerization isconducted in the presence of these compounds, no aggregation of polymerparticles formed occurs upon the polymerization, and the polymer formeddoes not adhere to the wall of a polymerization vessel, so that foamablemicrospheres can be stably produced while efficiently removing heatgenerated by the polymerization. These compounds are used in aproportion of generally 0.001 to 1 part by weight, preferably 0.01 to0.1 parts by weight per 100 parts by weight of the polymerizablemonomer. These polymerization aids will be described in detail in“Production process of foamable microspheres (II)” which will bedescribed subsequently.

(6) Suspension Polymerization

Although the order that the respective components are added to theaqueous dispersion medium is optional, water and the dispersionstabilizer, and optionally the auxiliary stabilizer and polymerizationaid are generally added to one another to prepare an aqueous dispersionmedium containing the dispersion stabilizer.

In the present invention, the polymerizable monomer, polymerizationinitiator and foaming agent are generally premixed to prepare apolymerizable mixture. As described above, specific methods fordispersing the polymerizable mixture (oily mixture) in the aqueousdispersion medium include {circle around (1)} the method in which theaqueous dispersion medium and the polymerizable mixture are continuouslyfed as separate streams at a fixed ratio into the continuous high-speed,high-shear type stirring and dispersing machine, and both arecontinuously stirred in the stirring and dispersing machine to dispersethe polymerizable mixture in the aqueous dispersion, and {circle around(2)} the method in which the aqueous dispersion medium and thepolymerizable mixture are poured into a dispersing tank, both arestirred in the dispersing tank to primarily disperse the polymerizablemixture in the aqueous dispersion medium, and the resultant primarydispersion is fed into the continuous high-speed, high-shear typestirring and dispersing machine to further continuously stir both in thestirring and dispersing machine, thereby dispersing the polymerizablemixture in the aqueous dispersion. Since the droplet diameter ofdroplets of the polymerizable mixture varies according to the change ofthe mixing ratio of the aqueous dispersion medium to the polymerizablemixture in the method {circle around (1)}, the method {circle around(2)} is preferred.

The suspension polymerization is generally conducted at a temperatureraised to 40 to 80° C. after deaerating the interior of the reactiontank or purged with an inert gas. After the suspension polymerization,the aqueous phase is removed by, for example, filtration, centrifugationor precipitation. The resultant foamable microspheres are dried as sucha comparatively low temperature that the foaming agent is gasified, asneeded.

Production Process of Foamable Microspheres (II)

The feature of the second production process according to the presentinvention resides in that in a process for producing foamablemicrospheres with a foaming agent enclosed in the shell of a polymerformed by subjecting a polymerizable mixture containing at least thefoaming agent and a polymerizable monomer to suspension polymerizationin an aqueous dispersion medium, the suspension polymerization of thepolymerizable mixture is conducted in the presence of a specifiedcompound. More specifically, the suspension polymerization of thepolymerizable mixture is conducted in the presence of at least onecompound selected from the group consisting of alkali metal nitrites,stanous chloride, stannic chloride, water-soluble ascorbic acids andboric acid.

The action mechanism of these compounds is not always clearly known atthe present stage. However, the presence of these compounds in thesuspension polymerization reaction system permits exhibiting such actionand effects that {circle around (1)} aggregation of polymer particlesformed can be prevented upon the polymerization, {circle around (2)} theadhesion of polymer scale to the wall of a polymerization vessel can beprevented, and {circle around (3)} a proportion of deformed particlesformed can be reduced to provide foamable microspheres even in particleshape in the form of a sphere. Since the prevention of the aggregationof the polymer particles permits preventing the viscosity of the slurryfrom increasing upon the polymerization, and the prevention of theadhesion of the polymer scale permits effectively removing heatgenerated by the polymerization, the polymerization reaction can bestably performed. As a result, foamable microspheres in the form of asphere, which can sharply foam, can be provided.

The foamable microspheres obtained by the production process accordingto the present invention cause uniform foaming. When the foaming stateof foamable microspheres is observed through, for example, a microscopeequipped with a hot stage while heating them, it is found that thefoamable microspheres obtained by the production process according tothe present invention foam at a stretch like popcorn to form uniformfoams. On the other hand, some of foamable microspheres obtained bysuspension polymerization in the absence of the specified compound mayinitiate foaming at a temperature lower than the foam initiatingtemperature by at least 10° C. or at least 30° C. in some cases comparedwith the foamable microspheres obtained by the production processaccording to the present invention. In addition, the foamablemicrospheres obtained by suspension polymerization in the absence of thespecified compound is observed greatly reducing their weight due to leakof the foaming agent even at a temperature lower than the foaminitiating temperature when their weight loss upon heating is.determinedby a thermobalance (TGA). Such weight loss takes place due to the leakof the foaming agent from the foamable microspheres before initiation offoaming. Therefore, the prescribed expansion ratio may not be achieved,and in extreme cases, no foaming occurs. When the process described inJapanese Patent Application Laid-Open No. 196813/1995 is applied to suchfoamable microspheres uneven in foaming, partial foaming occurs uponpreheating mixture of the foamable microspheres and a plasticizer. Sincefoamable microspheres expand to a degree of about 60 to 100 times byvolume upon foaming, the mixture with the plasticizer loses itsflowability even when foaming partially occurs, so that its pumpingbecomes infeasible.

(1) Polymerization Aid

The suspension polymerization of the polymerizable mixture is conductedin the presence of at least one compound selected from the groupconsisting of alkali metal nitrites, stanous chloride, stannic chloride,water-soluble ascorbic acids and boric acid according to the productionprocess of the present invention, whereby such inconveniences asdescribed above are solved, and foamable microspheres having excellentvarious properties can be stably provided.

Among the alkali metal nitrites, sodium nitrite and potassium nitriteare preferred from the viewpoints of easy availability and price.Examples of the ascorbic acids include ascorbic acid, metal salts ofascorbic acid and ascorbic esters. However, those soluble in water arepreferably used. The water-soluble ascorbic acids in the presentinvention means their solubility in water at 23° C. is at least 1 g/100cm³. Therefore, ascorbic acid and alkali metal salts thereof arepreferred. Among these, L-ascorbic acid (vitamin C), sodium ascorbateand potassium ascorbate are particularly preferably used from theviewpoints of easy availability, and action and effects. These compoundsare used in a proportion of generally 0.001 to 1 part by weight,preferably 0.01 to 0.1 parts by weight per 100 parts by weight of thepolymerizable monomer.

(2) Suspension Polymerization

In the production process according to the present invention, noparticular limitation is imposed on the foaming agent, polymerizablemonomer, other auxiliaries, etc., and those conventionally known may beused. More specifically, the production processes according to thepresent invention can be applied to the production of foamablemicrospheres of all types.

In the production process according to the present invention, thefoaming agent, polymerizable monomer, crosslinkable monomer,polymerization initiator, dispersion stabilizer, other auxiliaries,etc., which are used in “Production process of foamable microspheres(I)” as described above, may be suitably used. With respect to thecomposition of the polymerizable monomer as well, the compositionadopted in “Production process of foamable microspheres (I)” asdescribed above is preferably used.

Although the order that the respective components are added to theaqueous dispersion medium is optional, water and the dispersionstabilizer, and optionally the auxiliary stabilizer and polymerizationaid are generally added to a polymerization vessel to prepare an aqueousdispersion medium containing the dispersion stabilizer. In the presentinvention, at least one compound selected from the group consisting ofalkali metal nitrites, stanous chloride, stannic chloride, water-solubleascorbic acids and boric acid is added to the aqueous dispersion medium.The polymerizable monomer and the foaming agent may be separately addedto the aqueous dispersion medium to unite them in the aqueous dispersionmedium, thereby preparing a polymerizable mixture (oily mixture).However, they are generally added to the aqueous dispersion medium afterpremixing them. The polymerization initiator may be used by adding it tothe polymerizable monomer in advance. However, when prematurepolymerization must be prevented, for example, a mixture of thepolymerizable monomer and the foaming agent is added into the aqueousdispersion medium, and the polymerization initiator is added whilestirring the resultant mixture, thereby uniting them in the aqueousdispersion medium. Incidentally, mixing of the polymerizable mixture(oily mixture) with the aqueous dispersion medium may be conducted inanother container, and the resultant mixture may be stirred and mixedand then charged into the polymerization vessel.

Since the polymerizable mixture containing the foaming agent,polymerizable monomer, polymerization initiator, etc. forms an oil phasein the aqueous dispersion medium, fine droplets having a desired sizecan be formed by stirring and mixing it. Upon the stirring and mixing,conditions such ds the kind and the number of revolutions of thestirring machine are preset according to the desired particle diameterof the foamable microspheres. At this time, the conditions are selectedtaking the size and shape of the polymerization vessel, the presence ofa baffle, and the like into consideration. In the second productionprocess of the present invention, no particular limitation is imposed onthe stirring machine, and a homogenizer having high shearing force ispreferably used. It goes without saying that the first productionprocess of the present invention may also be applied to the secondproduction process to use the continuous high-speed, high-shear typestirring and dispersing machine.

The polymerization is generally conducted at 40 to 80° C. afterdeaerating the interior of the reaction vessel or purged with an inertgas. After the polymerization, the aqueous phase is removed by, forexample, filtration, centrifugation or precipitation. The resultantfoamable microspheres are dried as such a comparatively low temperaturethat the foaming agent is not gasified, as needed.

The particle diameter of the unformed foamable microspheres and theirparticle diameter after foaming may be varied over a wide range anddesigned on the basis of the nature required of the final product. Theaverage particle diameter of the foamable microspheres obtained by thepresent invention is generally 3 to 100 μm, preferably 5 to 50 μm in anunformed state. The content of the foaming agent is generally 5 to 30wt. %, preferably 10 to 25 wt. %. Foamable microspheres exhibitingvarious foaming behaviors can be produced by selecting the combinationof the polymerizable monomers used, controlling a compositional ratiotherebetween, and selecting the kind of the foaming agent.

Application Field

The foamable microspheres obtained by the present invention are used invarious fields after they are foamed (expanded) or as they are keptunformed. The foamable microspheres are used as, for example, fillersfor paints for automobiles, wallpaper, foaming agents for foaming ink(for applying relief patterns to T-shirts and the like),shrink-preventing agents, etc. making good use of, for example, theirexpanding ability. The foamable microspheres are used for the purpose ofreducing the weights of plastics, paints, various materials, etc.,making them porous and imparting various functionalities (for example,slip property, heat insulating property, cushioning property, soundinsulating property, etc.) making good use of their volume increase byexpansion. In particular, the foamable microspheres according to thepresent invention can be preferably used in the fields of paint,wallpaper and ink of which surface properties and smoothness arerequired.

EXAMPLES

The present invention will hereinafter be described more specifically bythe following Examples and Comparative Examples.

<Measuring Methods>

(1) Expansion Ratio

A foamable microsphere sample (0.7 g) is placed in a Geer oven andheated for 2 minutes at a prescribed temperature (foaming temperature)to foam it. The resultant foams are placed in a graduated cylinder tomeasure their volume. This volume is divided by the volume before thefoaming to regard the resultant value as an expansion ratio.

(2) Particle Diameter Distribution

Measured by means of a particle diameter distribution meter SALD-3000Jmanufactured by Shimadzu Corporation.

(3) ΔT

Showing a temperature difference between the temperature of a polymerslurry in a 1.5-liter polymerization vessel upon polymerization in thepolymerization vessel and the temperature of a hot water bath (amount ofhot water: 60 liters) in which the polymerization vessel has beenimmersed.

Production Process Using a Continuous High-speed, High-shear TypeStirring and Dispersing Machine

Example 1

A polymerization vessel (1.5 liters) equipped with a stirrer was chargedwith 16.5 g (41.3 g of a dispersion of silica having a solid content of40 wt. %) of colloidal silica, 1.65 g of (3.3 g in terms of a 50%solution) of a condensation product (acid number: 78 mg KOH/g) ofdiethanolamine and adipic acid, 169.8 g of common salt, 0.11 g of sodiumnitrite and water in such an amount that the total weight of thecontents amounts to 557 g, thereby preparing an aqueous dispersionmedium. Hydrochloric acid was added so as to keep the pH of the aqueousdispersion medium at 3.2.

On the other hand, a polymerizable mixture composed of 147.4 g ofacrylonitrile, 68.2 g of methacrylonitrile, 4.4 g of methylmethacrylate, 0.66 g of trimethylolpropane trimethacrylate, 26.2 g ofn-pentane and 15 g of petroleum ether was prepared (wt. % of monomercomponents=acrylonitrile/methacrylonitrile/methyl methacrylate=67/31/2).

Upon stirring and mixing of the polymerizable mixture with the aqueousdispersion medium, the aqueous dispersion medium and the polymerizablemixture were respectively stored in separate tanks as illustrated inFIG. 1, and they were continuously passed through a continuoushigh-speed, high-shear type stirring and dispersing machine (the numberof revolutions=2,500 rpm) at a fixed ratio from these tanks. The aqueousdispersion medium containing the fine droplets of the polymerizablemixture was then charged into a polymerization vessel (1.5 liters)equipped with a stirrer to conduct a reaction at 60° C. for 20 hours bymeans of a hot water bath. The resultant reaction product was filteredand washed with water repeatedly, and dried to obtain foamablemicrospheres having an average particle diameter of 27 μm and acoefficient of variation of 0.30%.

The expansion ratio of the foamable microspheres at a foamingtemperature, 170° C. was 58 times. When the foaming state of thefoamable microspheres was observed through a microscope equipped with ahot stage while heating them at a rate of 5° C./min, those foaming at atemperature not higher than 140° C. were scarcely observed. Accordingly,it is judged that foaming sharply occurs.

The polymerizable mixture and aqueous dispersion medium prepared abovewere stirred and mixed by means of a batch-wise high-speed, high-sheartype dispersing machine illustrated in FIG. 3 to form fine droplets ofthe polymerizable mixture. The aqueous dispersion medium containing thefine droplets of the polymerizable mixture was charged into apolymerization vessel (1.5 liters) equipped with a stirrer to conduct areaction at 60° C. for 20 hours by means of a hot water bath. Theresultant reaction product was filtered and washed with waterrepeatedly, and dried. As a result, foamable micrbspheres having anaverage particle diameter of 26 μm and a coefficient of variation of1.75% were obtained. The expansion ratio of the foamable microspheres ata foaming temperature, 170° C. was 50 times. When the foaming state ofthe foamable microspheres was observed through a microscope equippedwith a hot stage while heating them at a rate of 10° C./min, thosefoaming at a temperature not higher than 140° C. were scarcely observed.When the foaming state thereof was observed in more detail by changingthe heating rate to 5° C./min, however, some of them were observedfoaming at a temperature not higher than 140° C. Accordingly, it isunderstood that foamable microspheres more sharp in particle diameterdistribution are obtained by using the continuous high-speed, high-sheartype stirring and dispersing machine.

Example 2

Foamable microspheres were prepared in the same manner as in Example 1except that upon stirring and mixing of the polymerizable mixture withthe aqueous dispersion medium, the polymerizable mixture containing thefoaming agent and the polymerizable monomers was primarily dispersed inthe aqueous dispersion medium as illustrated in FIG. 2, the resultantprimary dispersion was passed through the continuous high-speed,high-shear type stirring and dispersing machine (the number ofrevolutions=2,500 rpm), and the suspension polymerization was thenconducted. The resultant reaction product was filtered and washed withwater repeatedly, and dried to obtain foamable microspheres having anaverage particle diameter of 26 μm and a coefficient of variation of0.37%.

The expansion ratio of the foamable microspheres at a foamingtemperature, 170° C. was 54 times. When the foaming state of thefoamable microspheres was observed through a microscope equipped with ahot stage while heating them at a rate of 5° C./min, those foaming at atemperature not higher than 140° C. were scarcely observed. Accordingly,it is judged that foaming sharply occurs.

Comparative Example 1

A polymerization vessel (1.5 liters) equipped with a stirrer was chargedwith 770 g of deionized water and 11 g of colloidal silica having asolid content of 40 wt. % to dissolve the silica in the deionized water.Further, hydrochloric acid was added to prepare an aqueous dispersionmedium having a pH of 3.5.

On the other hand, a polymerizable mixture composed of 123.2 g ofvinylidene chloride, 85.8 g of acrylonitrile, 11 g of methylmethacrylate, 0.33 g of trimethylolpropane trimethacrylate, 1.1 g of2,2′-azobis-2,4-dimethylvaleronitrile and 35.2 g of butane was prepared(wt. % of monomer components=vinylidene chloride/acrylonitrile/methylmethacrylate=59/39/5). This polymerizable mixture and the aqueousdispersion medium prepared above were then stirred and mixed by means ofa batch-wise high-speed, high-shear type dispersing machine illustratedin FIG. 3 to form fine droplets of the polymerizable mixture. Theaqueous dispersion medium containing the fine droplets was then chargedinto a polymerization vessel to conduct a reaction at 50° C. for 22hours. The resultant reaction product was filtered and washed with waterrepeatedly, and dried to obtain foamable microspheres having an averageparticle diameter of 13 μm and a coefficient of variation of 3.64%.

Example 3

Foamable microspheres were prepared in the same manner as in ComparativeExample 1 except that no batch-wise high-speed, high-shear typedispersing machine was used upon stirring and mixing of thepolymerizable mixture with the aqueous dispersion medium, the aqueousdispersion medium and the polymerizable mixture were respectively storedin separate tanks as illustrated in FIG. 1, they were continuouslypassed through a continuous high-speed, high-shear type stirring anddispersing machine (the number of revolutions=2,500 rpm) at a fixedratio, and the suspension polymerization was then conducted.

The resultant reaction product was filtered and washed with waterrepeatedly, and dried to obtain foamable microspheres having an averageparticle diameter of 14 μm and a coefficient of variation of 0.43%.

Comparative Example 2

A polymerization vessel (1.5 liters) equipped with a stirrer was chargedwith 770 g of deionized water and 11 g of colloidal silica having asolid content of 40 wt. % to dissolve the silica in the deionized water.Further, hydrochloric acid was added to prepare an aqueous dispersionmedium having a pH of 3.5.

On the other hand, a polymerizable mixture composed of 123.2 g ofacrylonitrile, 85.8 g of methyl methacrylate, 11 g of methyl acrylate,0.33 g of trimethylolpropane trimethacrylate, 1.1 g of2,2′-azobis-2,4-dimethylvaleronitrile and 35.2 g of isopentane wasprepared (wt. % of monomer components=acrylonitrile/methylmethacrylate/methyl acrylate=56/39/5).

This polymerizable mixture and the aqueous dispersion medium preparedabove were then stirred and mixed by means of a batch-wise high-speed,high-shear type dispersing machine to form fine droplets of thepolymerizable mixture. The aqueous dispersion medium containing the finedroplets was then charged into a polymerization vessel to conduct areaction at 50° C. for 22 hours. The resultant reaction product wasfiltered and washed with water repeatedly, and dried to obtain foamablemicrospheres having an average particle diameter of 12 μm and acoefficient of variation of 3.17%.

Example 4

Foamable microspheres were prepared in the same manner as in ComparativeExample 2 except that no batch-wise high-speed, high-shear typedispersing machine was used upon stirring and mixing of thepolymerizable mixture with the aqueous dispersion medium, the aqueousdispersion medium and the polymerizable mixture were respectively storedin separate tanks as illustrated in FIG. 1, they were continuouslypassed through a continuous high-speed, high-shear type stirring anddispersing machine (the number of revolutions=2,500 rpm) at a fixedratio, and the suspension polymerization was then conducted.

The resultant reaction product was filtered and washed with waterrepeatedly, and dried to obtain foamable microspheres having an averageparticle diameter of 16 μm and a coefficient of variation of 1.00%.

Production Process Using a Polymerization Aid

Comparative Example 3

A polymerization vessel (1.5 liters) equipped with a stirrer was chargedwith 16.5 g (41.3 g of a dispersion of silica having a solid content of40 wt. %) of colloidal silica, 1.65 g of (3.3 g in terms of a 50%solution) of a condensation product (acid number: 78 mg KOH/g) ofdiethanolamine and adipic acid, 169.8 g of common salt and water in suchan amount that the total weight of the contents amounts to 557 g,thereby preparing an aqueous dispersion medium. Hydrochloric acid wasadded so as to keep the pH of the aqueous dispersion medium at 3.2.

On the other hand, an oily mixture composed of 147.4 g of acrylonitrile,68.2 g of methacrylonitrile, 4.4 g of methyl methacrylate, 0.66 g oftrimethylolpropane trimethacrylate, 26.2 g of n-pentane and 15 g ofpetroleum ether was prepared (ratio by parts by weight:acrylonitrile/methacrylonitrile/methyl methacrylate=67/31/2). This oilymixture and the aqueous dispersion medium prepared above were stirredand mixed in a homogenizer to form fine droplets of the oily mixture.

The aqueous dispersion medium containing the fine droplets of the oilymixture was charged into a polymerization vessel (1.5 liters) equippedwith a stirrer to conduct a reaction at 60° C. for 20 hours by means ofa hot water bath. As a result, it was difficult to remove heat generatedby the reaction, and ΔT reached 2.7° C. The viscosity of the polymerslurry rapidly increased during the polymerization reaction, so that theflowability of the slurry was very deteriorated. The sifting ability ofthe slurry obtained after the polymerization was also poor. Manyaggregates were observed in the slurry, and the polymer adhered as scaleto the wall of the polymerization vessel. The resultant reaction productwas filtered and washed with water repeatedly, and dried to obtainfoamable microspheres having an average particle diameter of 30 μm and abulk density of 0.36 g/cm³. The expansion ratio of the foamablemicrospheres at a foaming temperature, 170° C. was 45 times.

When the foaming state of the foamable microspheres was observed througha microscope equipped with a hot stage while heating them at a rate of10° C./min, those foaming at a temperature not higher than 130° C. wereobserved in large numbers.

The viscosity of a 33.3 wt. % solution of the unformed foamablemicrospheres in diisononyl phthalate (hereinafter abbreviated as “DINP”)was as high as 1,600 centipoises. Incidentally, the reaction was scaledup to a 10-liter polymerization vessel in accordance with theformulation described above. As a result, it was impossible to controlthe reaction temperature.

Example 5

Foamable microspheres were prepared in the same manner as in ComparativeExample 3 except that 0.11 g of sodium nitrite were further added uponthe preparation of the aqueous dispersion medium. ΔT was as very smallas 0.2° C., and heat generated by the polymerization was able to befully removed even when the reaction was scaled up to a 10-literpolymerization vessel. The viscosity of the slurry upon thepolymerization in the 1.5-liter polymerization vessel was low, theflowability of the slurry was very good, and the sifting ability of theslurry thus obtained was also good. Occurrence of aggregates andadhesion of polymer scale to the wall of the polymerization vessel werescarcely observed. The resultant reaction product was filtered andwashed with water repeatedly, and dried to obtain foamable microsphereshaving an average particle diameter of 28 μm and a bulk density of 0.43g/cm³. The expansion ratio of the foamable microspheres at a foamingtemperature, 170° C. was 55 times. When the foaming state of thefoamable microspheres was observed through a microscope equipped with ahot stage while heating them at a rate of 10° C./min, those foaming at atemperature not higher than 140° C. were scarcely observed. Accordingly,it is judged that foaming sharply occurs. The viscosity of a 33.3 wt. %solution of the unformed foamable microspheres in DINP was as low as 720centipoises, and so the flowability of the solution was good.

Comparative Example 4

A polymerization vessel (1.5 liters) equipped with a stirrer was chargedwith 792 g of deionized water, and 39.6 g of magnesium chloridehexahydrate were added under stirring to dissolve it in the deionizedwater. To this solution, were added 0.044 g of Pelex OT-P (product ofKao Corporation, sodium dialkyl sulfosuccinate) and 23.8 g of sodiumhydroxide having a solid content of 25 wt. %, thereby preparing acolloidal dispersion of magnesium hydroxide. The pH of the aqueousdispersion (aqueous dispersion medium) was 9.8.

On the other hand, an oily mixture composed of 182.6 g of acrylonitrile,26.4 g of methyl methacrylate, 11 g of methyl acrylate, 0.44 g oftrimethylolpropane trimethacrylate, 1.1 g of2,2′-azobis-2,4-dimethylvaleronitrile and 39.6 g of pentane was prepared(ratio by parts by weight: acrylonitrile/methyl methacrylate/methylacrylate=83/12/5). This oily mixture and the aqueous dispersion mediumprepared above were then stirred and mixed by means of a homogenizer toform fine droplets of the oily mixture. The aqueous dispersion mediumcontaining the fine droplets was then charged into the polymerizationvessel to conduct a reaction at 57° C. for 20 hours.

ΔT was as great as 6° C., and it was impossible to control thepolymerization temperature due to insufficient removal of heat generatedby the polymerization when the reaction was scaled up to a 10-literpolymerization vessel. The viscosity of the slurry upon thepolymerization in the 1.5-liter polymerization vessel rapidly increased,so that the flowability of the slurry was very deteriorated. The siftingability of the slurry thus obtained was also poor. Many aggregates wereobserved in the slurry, and polymer scale adhered to the wall of thepolymerization vessel and the agitating blade.

The resultant reaction product was filtered and washed with waterrepeatedly, and dried to obtain foamable microspheres having an averageparticle diameter of 30 μm and a bulk density of 0.35 g/cm³. Theexpansion ratio of the foamable microspheres at a foaming temperature,170° C. was 39 times. The viscosity of a 33.3 wt. % solution of theunformed foamable microspheres in DINP was as high as 2,500 centipoises.When the foaming state of the foamable microspheres was observed througha microscope equipped with a hot stage while heating them at a rate of10° C./min, those foaming at a temperature not higher than 130° C. wereobserved in large numbers.

Example 6

Foamable microspheres were prepared in the same manner as in ComparativeExample 4 except that 0.11 g of boric acid were further added upon thepreparation of the aqueous dispersion (aqueous dispersion medium). ΔTbecame as small as 2° C., and heat generated by the polymerization wasable to be fully removed even when the reaction was scaled up to a10-liter polymerization vessel. The viscosity of the slurry upon thepolymerization in the 1.5-liter polymerization vessel was low, theflowability of the slurry was good, and the sifting ability of theslurry thus obtained was also good. Occurrence of aggregates andadhesion of polymer scale to the wall of the polymerization vessel werescarcely observed.

The resultant reaction product was filtered and washed with waterrepeatedly, and dried to obtain foamable microspheres having an averageparticle diameter of 28 μm and a bulk density of 0.38 g/cm³. Theexpansion ratio of the foamable microspheres at a foaming temperature,170° C. was 49 times. When the foaming state of the foamablemicrospheres was observed through a microscope equipped with a hot stagewhile heating them at a rate of 10° C./min, those foaming at atemperature not higher than 140° C. were scarcely observed. Accordingly,it is judged that foaming sharply occurs. The viscosity of a 33.3 wt. %solution of the unformed foamable microspheres in DINP was as low as1,300 centipoises, and so marked improvement in flowability wasrecognized.

Comparative Example 5

A polymerization vessel (1.5 liters) equipped with a stirrer was chargedwith 770 g of deionized water and 11 g of colloidal silica having asolid content of 40 wt. % to dissolve the silica in the deionized water.Further, hydrochloric acid was added to prepare an aqueous dispersionmedium having a pH of 3.5.

On the other hand, an oily mixture composed of 123.2 g of vinylidenechloride, 85.8 g of acrylonitrile, 11 g of methyl methacrylate, 0.33 gof trimethylolpropane trimethacrylate, 1.1 g of2,2′-azobis-2,4-dimethylvaleronitrile and 35.2 g of butane was prepared(ratio by parts by weight: vinylidene chloride/acrylonitrile/methylmethacrylate=56/39/5). This oily mixture and the aqueous dispersionmedium prepared above were stirred and mixed in a homogenizer to formfine droplets of the oily mixture. The aqueous dispersion mediumcontaining the fine droplets was then charged into the polymerizationvessel to conduct a reaction at 50° C. for 22 hours.

ΔT was as great as 7° C., and it was impossible to control thepolymerization temperature due to insufficient removal of heat generatedby the polymerization when the reaction was scaled up to a 10-literpolymerization vessel. The viscosity of the slurry upon thepolymerization in the 1.5-liter polymerization vessel rapidly increased,so that the flowability of the slurry was very deteriorated. The siftingability of the slurry thus obtained was also poor. Many aggregates wereobserved in the slurry, and polymer scale adhered to the wall of thepolymerization vessel and the agitating blade. The resultant reactionproduct was filtered and washed with water repeatedly, and dried toobtain foamable microspheres having an average particle diameter of 14μm. The foamable microspheres were sifted by a 200-mesh sieve (sieveopening: 75 μm). As a result, the amount of the foamable microspheresremaining on the mesh was 5 wt. %. The expansion ratio of the foamablemicrospheres at a foaming temperature, 120° C. was 40 times.

Example 7

Foamable microspheres were prepared in the same manner as in ComparativeExample 5 except that 0.088 g of stannic chloride were further addedupon the preparation of the aqueous dispersion medium. ΔT became assmall as 1.5° C., and heat generated by the polymerization was able tobe fully removed even when the reaction was scaled up to a 10-literpolymerization vessel. The viscosity of the slurry upon thepolymerization in the 1.5-liter polymerization vessel was low, theflowability of the slurry was good, and the sifting ability of theslurry thus obtained was also good. Occurrence of aggregates andadhesion of polymer scale to the wall of the polymerization vessel werescarcely observed. The resultant reaction product was filtered andwashed with water repeatedly, and dried to obtain foamable microsphereshaving an average particle diameter of 15 μm. The foamable microsphereswere sifted by a 200-mesh sieve (sieve opening: 75 μm). As a result, theamount of the foamable microspheres remaining on the mesh was as verysmall as 0.1 wt. % or less. The expansion ratio of the foamablemicrospheres at a foaming temperature, 120° C. was 50 times.

Comparative Example 6

Foamable microspheres were prepared in the same manner as in ComparativeExample 1 except that 1.1 g of polyvinyl pyrrolidone having a molecularweight of 10,000 were added in place of the condensation product ofdiethanolamine and adipic acid upon the preparation of the aqueousdispersion medium. ΔT was as great as 3° C., and it was impossible tocontrol the polymerization temperature due to insufficient removal ofheat generated by the polymerization when the reaction was scaled up toa 10-liter polymerization vessel. The viscosity of the slurry upon thepolymerization in the 1.5-liter polymerization vessel rapidly increased,so that the flowability of the slurry was very deteriorated. The siftingability of the slurry thus obtained was also poor. Many aggregates wereobserved in the slurry, and polymer scale adhered to the wall of thepolymerization vessel and the agitating blade.

The resultant reaction product was filtered and washed with waterrepeatedly, and dried to obtain foamable microspheres having an averageparticle diameter of 32 μm and a bulk density of 0.36 g/cm³. Theexpansion ratio of the foamable microspheres at a foaming temperature,170° C. was 42 times. The viscosity of a 33.3 wt. % solution of theunformed foamable microspheres in DINP was as high as 2,700 centipoises.When the foaming state of the foamable microspheres was observed througha microscope equipped with a hot stage while heating them at a rate of10° C./min, those foaming at a temperature not higher than 130° C. wereobserved in large numbers.

Example 8

Foamable microspheres were prepared in the same manner as in ComparativeExample 6 except that 0.13 g of sodium nitrite were further added uponthe preparation of the aqueous dispersion medium. ΔT became as verysmall as 0.3° C., and heat generated by the polymerization was able tobe fully removed even when the reaction was scaled up to a 10-literpolymerization vessel. The viscosity of the slurry upon thepolymerization in the 1.5-liter polymerization vessel was low, theflowability of the slurry was good, and the sifting ability of theslurry thus obtained was also good. Occurrence of aggregates andadhesion of polymer scale to the wall of the polymerization vessel werescarcely observed.

The resultant reaction product was filtered and washed with waterrepeatedly, and dried to obtain foamable microspheres having an averageparticle diameter of 31 μm and a bulk density of 0.42 g/cm³. Theexpansion ratio of the foamable microspheres at a foaming temperature,170° C. was 57 times. The viscosity of a 33.3 wt. % solution of theunformed foamable microspheres in DINP. was as low as 800 centipoises,and so the flowability of the solution was good. When the foaming stateof the foamable microspheres was observed through a microscope equippedwith a hot stage while heating them at a rate of 10° C./min, thosefoaming at a temperature not higher than 140° C. were not very observed.Accordingly, it is judged that foaming sharply occurs.

Example 9

Foamable microspheres were prepared in the same manner as in Example 8except that 0.3 g of L-ascorbic acid (vitamin C) were added in place ofsodium nitrite upon the preparation of the aqueous dispersion medium.

ΔT became as very small as 0.3° C., and heat generated by thepolymerization was able to be fully removed even when the reaction wasscaled up to a 10-liter polymerization vessel. The viscosity of theslurry upon the polymerization in the 1.5-liter polymerization vesselwas low, the flowability of the slurry was good, and the sifting abilityof the slurry thus obtained was also good. Occurrence of aggregates andadhesion of polymer scale to the wall of the polymerization vessel werescarcely observed.

The resultant reaction product was filtered and washed with waterrepeatedly, and dried to obtain foamable microspheres having an averageparticle diameter of 32 μm and a bulk density of 0.43 g/cm³. Theexpansion ratio of the foamable microspheres at a foaming temperature,170° C. was 62 times. The viscosity of a 33.3 wt. % solution of theunformed foamable microspheres in DINP was as low as 750 centipoises,and so the flowability of the solution was good. When the foaming stateof the foamable microspheres was observed through a microscope equippedwith a hot stage while heating them at a rate of 10° C./min, thosefoaming at a temperature not higher than 140° C. were not very observed.Accordingly, it is judged that foaming sharply occurs.

Industrial Applicability

According to the present invention, there can be provided foamablemicrospheres which have a spherical particle shape, are extremely sharpin particle diameter distribution and low in viscosity when preparedinto a slurry, and are capable of sharply foaming to provide uniformfoams. The foamable microspheres obtained by the production processaccording to the present invention have an extremely sharp particlediameter distribution of at most 1.50% in terms of the coefficient ofvariation of the particle diameter distribution and hence can sharplyfoam to form uniform foams due to their low contents of coarse particlesand minute particles.

According to the production processes of the present invention,aggregation of polymer particles formed is prevented upon thepolymerization, and the polymer formed does also not adhere to the wallof a polymerization vessel, so that heat generated by the polymerizationcan be efficiently removed, and moreover high-quality foamablemicrospheres can be stably produced. The foamable microspheres obtainedby the production processes of the present invention are even inparticle shape in the form of a sphere and can sharply foam due to theirlow contents of aspherical particles and aggregated particles to formuniform foams.

What is claimed is:
 1. Foamable microspheres with a foaming agentenclosed in the shell of a polymer, wherein the average particlediameter of the microspheres is within a range of 3 to 100 μm, and thecoefficient of variation of the particle diameter distribution thereofis at most 1.50%.
 2. The foamable microspheres according to claim 1,wherein the shell of the polymer is formed from a vinylidene chloridecopolymer or a (meth)acrylonitrile copolymer.
 3. The foamablemicrospheres according to claim 1, which are obtained by subjecting apolymerizable mixture containing at least the foaming agent and apolymerizable monomer to suspension polymerization in an aqueousdispersion medium.
 4. A process for producing foamable microspheres witha foaming agent enclosed in the shell of a polymer formed by subjectinga polymerizable mixture containing at least the foaming agent and apolymerizable monomer to suspension polymerization in an aqueousdispersion medium, the process comprising feeding the aqueous dispersionmedium and the polymerizable mixture into a continuous high-speed,high-shear type stirring and dispersing machine, continuously stirringboth in the stirring and dispersing machine so as to disperse thepolymerizable mixture in the aqueous dispersion medium, and then pouringthe resultant dispersion into a polymerization tank to conductsuspension polymerization in the polymerization tank to produce foamablemicrospheres having a coefficient of variation of the particle diameterdistribution of at most 1.50%.
 5. The production process according toclaim 4, wherein in the step of feeding the aqueous dispersion mediumand the polymerizable mixture into the continuous high-speed, high-sheartype stirring and dispersing machine, the aqueous dispersion medium andthe polymerizable mixture are continuously fed as separate streams at afixed ratio into the continuous high-speed, high-shear type stirring anddispersing machine.
 6. The production process according to claim 4,wherein in the step of feeding the aqueous dispersion medium and thepolymerizable mixture into the continuous high-speed, high-shear typestirring and dispersing machine, the aqueous dispersion medium and thepolymerizable mixture are poured into a dispersing tank, both arestirred in the dispersing tank to primarily disperse the polymerizablemixture in the aqueous dispersion medium, and the resultant primarydispersion is then fed into the continuous high-speed, high-shear typestirring and dispersing machine.
 7. The production process according toclaim 4, wherein in the step of continuously stirring the aqueousdispersion medium and the polymerizable mixture in the continuoushigh-speed, high-shear type stirring and dispersing machine so as todisperse the polymerizable mixture in the aqueous dispersion medium, thecontinuous high-speed, high-shear type stirring and dispersing machineis rotated at the number of revolutions within a range of 1,400 to14,000 rpm.
 8. The production process according to claim 4, wherein theaqueous dispersion medium contains water, a dispersion stabilizer and atleast one compound selected from the group consisting of alkali metalnitrites, stannous chloride, stannic chloride, water-soluble ascorbicacids and boric acid as a polymerization aid.
 9. The production processaccording to claim 4, wherein foamable microspheres having an averageparticle diameter within a range of 3 to 100 μm and a coefficient ofvariation of the particle diameter distribution of at most 1.50% areobtained by the suspension polymerization.
 10. The production processaccording to claim 4, wherein the polymerizable monomer is a monomermixture containing (a) 30 to 95 wt. % of vinylidene chloride and (b) 5to 70 wt. % of at least one monomer selected from the group consistingof acrylonitrile, methacrylonitrile, acrylates, methacrylates, styreneand vinyl acetate.
 11. The production process according to claim 10,wherein the monomer mixture contains (a) 40 to 80 wt. % of vinylidenechloride, (b1) 19 to 50 wt. % of at least one monomer selected from thegroup consisting of acrylonitrile and methacrylonitrile, and (b2) 1 to20 wt. % of at least one monomer selected from the group consisting ofacrylates and methacrylates.
 12. The production process according toclaim 4, wherein the polymerizable monomer is a monomer mixturecontaining (c) 51 to 95 wt. % of at least one monomer selected from thegroup consisting of acrylonitrile and methacrylonitrile and (d) 5 to 49wt. % of at least one monomer selected from the group consisting ofvinylidene chloride, acrylates, methacrylates, styrene and vinylacetate.
 13. The production process according to claim 12, wherein themonomer mixture contains (c) 51 to 95 wt. % of at least one monomerselected from the group consisting of acrylonitrile andmethacrylonitrile, (d1) 1 to 40 wt. % of vinylidene chloride, and (d2) 1to 48 wt. % of at least one monomer selected from the group consistingof acrylates and methacrylates.
 14. The production process according toclaim 4, wherein the polymerizable monomer is a monomer mixturecontaining (e) 70 to 95 wt. % of at least one monomer selected from thegroup consisting of acrylonitrile and methacrylonitrile, and (f) 5 to 30wt. % of at least one monomer selected from the group consisting ofacrylates and methacrylates.
 15. The production process according toclaim 14, wherein the monomer mixture contains (e1) 55 to 75 wt. % ofacrylonitrile, (e2) 20 to 40 wt. % of methacrylonitrile, and (f) 1 to 10wt. % of at least one monomer selected from particles formed upon thepolymerization while preventing the polymer formed from adhering to thewall of a polymerization vessel and efficiently removing heat generatedby the polymerization. The foamable microspheres obtained according tothis production process can sharply foam to provide uniform foams due totheir low contents of aspherical particles and aggregated particles. 16.A process for producing foamable microspheres with a foaming agentenclosed in the shell of a polymer formed by subjecting a polymerizablemixture containing at least the foaming agent and a polymerizablemonomer to suspension polymerization in an aqueous dispersion medium,the process comprising conducting the suspension polymerization of thepolymerizable mixture in the presence of at least one compound selectedfrom the group consisting of alkali metal nitrites, stannous chloride,stannic chloride, water-soluble ascorbic acids and boric acid.
 17. Theproduction process according to claim 16, wherein the alkali metalnitrite is sodium nitrite or potassium nitrite.
 18. The productionprocess according to claim 16, wherein the water-soluble ascorbic acidis ascorbic acid, sodium ascorbate or potassium ascorbate.
 19. Theproduction process according to claim 16, wherein the compound iscontained in the aqueous dispersion medium in a proportion of 0.001 to 1part by weight per 100 parts by weight of the polymerizable monomer. 20.The production process according to claim 16, wherein the polymerizablemonomer is a monomer mixture containing (a) 30 to 95 wt. % of vinylidenechloride and (b) 5 to 70 wt. % of at least one monomer selected from thegroup consisting of acrylonitrile, methacrylonitrile, acrylates,methacrylates, styrene and vinyl acetate.
 21. The production processaccording to claim 20, wherein the monomer mixture contains (a) 40 to 80wt. % of vinylidene chloride, (b1) 19 to 50 wt. % of at least onemonomer selected from the group consisting of acrylonitrile andmethacrylonitrile, and (b2) 1 to 20 wt. % of at least one monomerselected from the group consisting of acrylates and methacrylates. 22.The production process according to claim 16, wherein the polymerizablemonomer is a monomer mixture containing (c) 51 to 95 wt. % of at leastone monomer selected from the group consisting of acrylonitrile andmethacrylonitrile and (d) 5 to 49 wt. % of at least one monomer selectedfrom the group consisting of vinylidene chloride, acrylates,methacrylates, styrene and vinyl acetate.
 23. The production processaccording to claim 22, wherein the monomer mixture contains (c) 51 to 95wt. % of at least one monomer selected from the group consisting ofacrylonitrile and methacrylonitrile, (d1) to 40 wt. % of vinylidenechloride, and (d2) 1 to 48 wt. % of at least one monomer selected fromthe group consisting of acrylates and methacrylates.
 24. The productionprocess according to claim 16, wherein the polymerizable monomer is amonomer mixture containing (e) 70 to 95 wt. % of at least one monomerselected from the group consisting of acrylonitrile andmethacrylonitrile, and (f) 5 to 30 wt. % of at least one monomerselected from the group consisting of acrylates and methacrylates. 25.The production process according to claim 24, wherein the monomermixture contains (e1) 55 to 75 wt. % of acrylonitrile, (e2) 20 to 40 wt.% of methacrylonitrile, and (f) 1 to 10 wt. % of at least one monomerselected from the group consisting of acrylates and methacrylates.